Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

The second session of the workshop “Searching for Life across Space and Time” was moderated by Bethany Ehlmann of the California Institute of Technology (Caltech) and Britney Schmidt of the Georgia Institute of Technology. Several solar system bodies may have had or may still have habitable environments on the surface, in liquids (both on the surface and underground), or in the atmosphere. The talks in this session focused mainly on Mars and the ocean moons, predominantly Enceladus of Saturn and Europa of Jupiter.

John Grotzinger of Caltech began his talk by thinking about Mars as a global system. To illustrate this, he showed a figure made by the session moderator, Ehlmann, showing a timeline of water-related environments on Mars (see Figure 2.1).

Martian Geological Record

Starting in the Noachian era (~4.1 billion years [Gyr] ago to ~3.7 Gyr ago), there could have been a mixture of aqueous surface environments including hot springs, lakes, and rivers. However, due to planetary processes, like the loss of a geodynamo, which allowed the solar wind to erode away the atmosphere, the surface water environment eventually disappeared. However, it might have come back periodically in pulses (Figure 2.1) that produced a range of elements, minerals, and salts that provide a geological history of Mars. The details of this story, such as the temporal boundaries and the abundance of surface water, are disputed. Grotzinger’s main point was that the Curiosity rover mission provides ample evidence that surface water existed into much younger periods of time than previously thought and that the early to middle Hesperian environment is more favorable for habitability than previously regarded. (The Hesperian era began ~3.7 Gyr ago.)

Grotzinger then suggested that there are other aspects of our understanding of ancient Mars that could use some rethinking as well, and these will in turn clarify the question of habitability. One line of thought is that Mars is a volcanic planet. While true, layered rocks are also suggestive of sedimentary geologies, and sedimentary basins are chemical reactors converting heat and fluid flows into aqueous minerals, as shown by Curiosity, the Mars Science Laboratory mission rover. Earth’s sedimentary rock, Grotzinger said, is an archive of Earth’s earliest

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

FIGURE 2.1 Water-related environments and alteration minerals in martian history. SOURCE: Adapted by permission from Macmillan Publishers Ltd.: B.L. Ehlmann, J.F. Mustard, S.L. Murchie, J.-P. Bibring, A. Meunier, A.A. Fraeman, and Y. Langevin, 2011, Subsurface water and clay mineral formation during the early history of Mars, Nature 479(7371):53-60, copyright 2011; presented in John Grotzinger, California Institute of Technology, “Habitable Environments of Ancient Mars; Deciphering the Rock Record,” presentation to the Workshop on Searching for Life across Space and Time, December 5, 2016.

biosphere. The primary feature ensuring that the record of that earlier biosphere has been preserved is silica-rich sedimentary rocks. Another area needing rethinking is seeing Mars as a glacial planet. Opposed to this is the fact that no glacial landforms have been encountered by rovers on the martian surface, and sedimentary deposits lack glacial features. Mars, he said, was apparently warm and wet enough for liquid water to be stable on Mars for 104 to 106 years. Another line of thought that needs to be revisited, according to Grotzinger, is that the transition from the Noachian era to the Hesperian era (boundary at ~3.7 Gyr ago) was a global acidification event. Supporting this is the finding that Meridiani Planum (visited by the second Mars Exploration rover, Opportunity) was generally acidic. Challenging this is the fact that Gale Crater (visited by Curiosity) was, in general, pH neutral. Furthermore, the largest river systems on Mars, which spanned the Noachian and Hesperian eras, produced only clays. Additionally, Grotzinger said, when considering biomarker preservation, texture and petrogenesis are just as important as the mineralogy. He said that we need more small and cheap rovers to visit all these various places.

Gale Crater

The Curiosity rover landed in Gale Crater. In the center of Gale Crater is Mt. Sharp. Grotzinger then showed a geological cross-section of the crater from the central peak to the northern rim. He wanted the audience to appreciate the topography, which shows erosion of the sedimentary deposits that once filled it. Curiosity drove across several geological boundaries in its journey. He said that looking at this mountain is like looking at the layered history of the Grand Canyon. He then showed a view of the crater with plotted results from orbital spectroscopy. Curiosity first traveled across rocks that were mostly covered with dust and that had revealed no minerals from orbit. However, the first hole drilled showed >20 percent clay, which means that much more of Mars may be composed of hydrated

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

minerals than is observed by spectroscopy from orbit. Curiosity then explored the stratigraphically younger and topographically higher Murray formation. This unit showed patches of different materials like silica, hematite, clay, and sulfate from orbit, but when drilled, it again showed a rich bounty of hydrated clays and other hydrated minerals. These minerals exist at abundances much higher than what was predicted from orbit (see Figure 2.2). Curiosity measured a stratigraphic column—layered rocks deposited as a function of time.

Curiosity rover data show that it landed on ancient conglomerates, riverbeds, channels, and rivers with gravelly sandstones. The rover also discovered features interpreted to represent ancient deltas. On Mars, you see river deposits passing into deltas and then on into lake deposits that are strikingly similar to what you see on Earth. This lead Grotzinger to believe that there were long-lived lakes on Mars. He said, however, that the persistence of lakes does not matter critically to habitability. Even when the lake’s surface dries out, there is still a habitable, aqueous environment below the surface. They imagine that the basin filled up with alternating lake deposits and maybe some dry deposits. Wind then eroded some of it away and left behind a mountain in the center.

As Curiosity moved up the mountain, it drilled different deposits in progressively younger stratigraphic positions (see Figure 2.2). The Chemical and Mineralogy X-ray Diffraction (CheMin) instrument used X-ray diffraction to examine the lake deposit sample and compared it to Gale soil samples representing primary igneous compositions. At Yellowknife Bay, compared to the soils, the drilled lake deposits showed that igneous minerals have been altered into other minerals, mostly an iron-magnesium clay mineral, but also some magnetite. This looks similar to the results of serpentinization, demonstrating that sedimentary basins are important chemical reactors, favorable

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

for microbial habitability. Samples drilled at Yellowknife Bay also revealed a statistically significant quantity of the reduced organic compound chlorobenzene, indicating that this geological environment was favorable for the preservation of organic compounds.

The Curiosity rover started in Yellowknife Bay in the crater basin and drove partway up Mt. Sharp. Along the way, it drilled many holes and analyzed their composition (see Figure 2.2). At the bottom of the stratigraphic sequence, there were chlorobenzene molecules, clay, and magnetite. A younger lake deposit had less magnetite, but Curiosity started to pick up some hematite and a little bit of jarosite that hinted at minor acidity. Moving farther up through the stratigraphy, the lake deposit changed composition again, losing all evidence of acidity and instead increasing magnetite along with a striking amount of both crystalline and amorphous silica. Another rock layer higher up had even more crystalline silica, along with magnetite and minor residual igneous minerals. More recent samples higher up the mountain show a lot of clay and some hematite, but no magnetite. Grotzinger said that this shows that the sedimentary basin acts as a chemical reactor. Primary igneous minerals are being converted into different minerals under different chemical circumstances, which he thinks is pretty exciting for habitability. These compositions might have been able to support several different metabolic pathways.

Grotzinger then showed an image of a striking rock, a very fine-grained chert composed of ~73 percent SiO2 with a millimeter thick lamination. The rock has a small amount of igneous minerals left (mostly plagioclase), some magnetite, opal CT, and a lot of amorphous materials, probably opal A. All of the mafic materials are gone. Instead, there is a lot of crystalline silica, including an exotic polymorph called tridymite. This, Grotzinger thinks, was likely transported from a felsic volcanic rock. This rock is very important because it is compositionally very similar to early rocks on Earth, which can contain microfossils. Silica is a great material that can survive through a number of geological processes, including, in some cases, thermal metamorphism. There is very strong evidence that this silica was created by primary enrichment, increasing its relevance to preservation of potential biological materials.

Grotzinger then discussed work by Joel Hurowitz on evidence for an ancient redox-stratified lake in Gale Crater. Certain areas of the lake have an abundance of oxidants, which they interpret as ultraviolet (UV) photolysis of water that created oxygen. Meanwhile, reduced iron percolated through the martian crust (i.e., groundwater seeped into the lake). When the level of oxidants exceeded the demand from reduced iron, the oxidants in the water then reacted with reduced iron, which caused the precipitation of hematite. With a little evaporation, some sulfate salts could have been produced as well. The silica-rich rock has a very different story though. In areas of the lake where the oxidant concentration did not exceed the reduced iron, magnetite was created instead. This means that there were multiple oxidation states in the ancient lake; even the lake itself was chemically stratified. This is very important for microbial habitability, which depends on redox gradients.

Grotzinger then said that new research has shown the possibility that the origin of life on Mars could have occurred on the surface. UV radiation could drive some of the chemistry. All that is needed is hydrogen cyanide and hydrogen sulfide, both of which are present on Mars. Gale Crater, he said, gives an opportunity to look at both environments: a long-lived environment possibly thermally warm enough for olivine to dissolve into and maybe even to allow a pathway towards hydrogen production or, alternatively, surface waters that could proceed with a different molecular chemistry.

Grotzinger finished by talking about groundwater. As Curiosity works its way up Mt. Sharp, it is finding fractures that cut across the sedimentary rock that are full of sulfate minerals. The Chemistry and Camera (Chem-Cam) instrument, however, is showing that the fractures are becoming increasingly enriched with boron, meaning that boron may be present as a trace component of other minerals, or perhaps present as amorphous compounds.

Audience Participation

A member of the audience asked if there was any evidence of carbonate minerals being formed and where the magnesium that was leached from these salts was going. Another audience member said that the question of where the carbonates and magnesium are going may be two separate questions. She said that the magnesium is

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

going into carbon in two places on Mars: the Nili Fossae northeast surface region and the Comanche outcrop. Both have magnesium from olivine going into carbonate, but the Nili Fossae region also has some magnesium going into clay minerals. In the Gale Crater, it seems like the magnesium is primarily going into phyllosilicate minerals, but also maybe some sulfates.

Because no rover has encountered glacial features on Mars, one audience member asked if this was just a site selection issue, considering that some geomorphology implies glacial features. Grotzinger admitted that that could be the case, but he also said that the sedimentary record doesn’t provide any evidence for glacial deposits. He went on to say that he’s sure glacial features are there, but glaciers probably aren’t the dominant paradigm on Mars.

According to a workshop participant, recent origin-of-life work published in Nature showed that every detail Grotzinger talked about, even olivines remaining in the residual sedimentary rocks eroding from an igneous facies, is exactly what is needed to go from formaldehyde, generated by the photochemical decomposition of carbon dioxide in the atmosphere, all the way to RNA. The audience member said that he just published a paper showing that opal CT, which was in one of the martian facies, absorbs oligomeric RNA and all of its intermediate steps. He then asked why Grotzinger didn’t mention two species, phosphate minerals and borate minerals. Grotzinger said that they do not detect any borate minerals. However, they do see phosphate, with fluorapatite being the dominant phase. They think it’s an igneous mineral, but there is phosphorus there. The silica enrichment they see is associated with the retention of phosphorus, which supports a pH-neutral body of water. This is because, if all the igneous minerals were being dissolved at low acidity, it should have been one of the first minerals to dissolve, but they are still seeing it anyway.

A workshop participant then briefly explained that, if intermittent, wet-dry cycles were good for biochemistry, and that the lack of a significant martian moon is an advantage. Earth has glaciation with just a 2° wobble in obliquity, while Mars can move from 10° to 50° and back over just tens of thousands of years.

Underneath the red, highly oxidized martian surface, one audience member said, there is a gray, likely reducing, material underneath. Hearkening back to the earlier talks about how life likes to use redox disequilibria, he asked how this boundary near the martian surface could contribute to habitability. Grotzinger said that the highest altitude drill samples on Mt. Sharp no longer show a gray subsurface. They are red throughout, which means that Gale preserves multiple oxidation states.

An audience member then went back to a previous point on phosphate. He said that there isn’t good information on soluble phosphate using X-ray diffraction from CheMin. Elemental analyses, however, do show that phosphorus enrichments are usually accompanied by calcium, and they likely are soluble. Changing topics to salts, he said that as long as there are lakes, the magnesium sulfate salts are not too concentrated for potential life. Only when the salt becomes an evaporite does the water activity become inconsistent with life. Additionally, he has done work showing that many organisms can tolerate high magnesium sulfate levels.

To one workshop participant, martian meteorites are interesting because you can look at the mass-independent fractions of the isotopes of sulfur and of oxygen (i.e., the fractions are not in proportion to the mass of the respective isotopes) in the sulfates, carbonates, and water in the host rock. It therefore looks like a lot of the sulfates are photochemically processed—a known pathway that circumvents mass-dependence. The water might be photochemically processed too because the oxygen in the water is also found to be mass-independent, but it isn’t in equilibrium with the sulfate and carbonate that it is found with. Grotzinger replied that a complex mass spectrometer could make that measurement. He then said that it would be a good idea to land in a place with lots of sulfate and to perform that measurement.

A member of the audience then asked Grotzinger which locations on Mars he would most like to land the aforementioned small “boutique” rover on. He said that he would first like to go to a carbonate site. Another place he thought would be a good idea is somewhere in Valles Marineris. Building off this question, another audience member then asked which new instruments Grotzinger would want on it. Grotzinger said that he would really like to see a rover with an imaging spectrometer land on a place with extreme mineral diversity. He also said that a laser Raman spectrometer would be great to have.

The last question asked of Grotzinger was when we were going to drill deep on Mars. Another audience member said that ExoMars, launching in 2020, would drill down 2 m into the martian surface.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Kevin Hand of the NASA Jet Propulsion Laboratory (JPL) began his talk showing a graphic of all robotic missions, successful and failed, to all bodies of the solar system. One of the most remarkable discoveries from these missions, Hand said, is that at least six worlds beyond Earth likely harbor subsurface, liquid water oceans: Europa, Ganymede, and Callisto of Jupiter; Enceladus and Titan of Saturn; and possibly Triton of Neptune. Additionally, Titan has an atmosphere and surface lakes of hydrocarbon liquids. Triton’s ocean may have some ammonia mixed in as well. Hand said that Alan Stern of the Southwest Research Institute, and the principal investigator of the New Horizons mission to Pluto, could make a very strong case for adding Pluto to the list of ocean worlds. Hand said that he would probably agree.

Hand then showed these moons on a grid with potential geophysical properties and processes to illustrate which moons are the most interesting in the context of searching for life beyond Earth (see Table 2.1). In particular, he emphasized the column showing which moons have oceans in contact with rock; a condition that, to the best of our knowledge, only exists on Europa and Enceladus. Europa’s ocean has likely been persistently habitable for most of the history of the solar system. Enceladus might also have survived with an ocean for the lifetime of the solar system, although a recent paper has argued that Enceladus is only 100 Myr old and was formed by a Kuiper belt object colliding with a body in the Saturnian system.

What really motivates Hand is the prospect that one of these bodies independently gave rise to life that is still extant. As a point of comparison, Hand said that the search for life on Mars is of great importance, but that the current strategy of searching for ancient life in the rock record of Mars would, if successful, leave many key questions unanswered. What is the fundamental biochemistry? How did that life originate, and was it from an independent, second origin? Was it seeded from life on Earth, or did Mars seed Earth? Answering these kinds of questions requires samples of life that go well beyond what is preserved in rocks billions of years old. The discovery of extant life in an ocean world would allow for a detailed study of life and its biochemistry that would not be possible from martian microfossils. Potential martian life would also be more likely to have been delivered from Earth (i.e., panspermia) than for the outer solar system’s icy bodies. For example, out of 600 million rocks produced by an asteroid collision with Earth, only ~30 to 100 rocks would land on Europa, and only ~3 to 20 on Titan, both with impact velocities >10 km/s, which would likely destroy any life transported within the rocks.1 If DNA were found on these bodies, it would strongly suggest that there is an evolutionary chemical convergence towards using DNA as the information storage molecules for life. This would also help us understand how life originated on Earth. Since the icy ocean worlds do not have continents or tide pools, a discovery of life there would argue against a primordial soup origin on Earth and in favor of a hydrothermal or even an icy origin. Conversely, were life not to be found within ocean worlds, one might be inclined to favor models for the origin of life that include continents and tidal pools. Either way, much about life beyond Earth and how life on Earth may have originated could be learned.

Liquid Water

Liquid water is the most important aspect of these bodies in terms of habitability. Hand said that the combined water volume in the icy moons, based on conservative estimates, is about 30 times higher than that of Earth. In the Jovian system, the liquid water is maintained through tidal heating and some radiogenic decay. Tidal energy dissipation would usually fade away as the orbits circularize, but the Laplace resonance of the interior three Jovian moons (Io, Europa, and Ganymede) forces an eccentricity that can maintain long-term tidal heating. This is a powerful heating source. While Earth’s average surface flux (for only the seafloor) is 60 to 80 mW/m2, Europa’s possible range is 10 to 800 mW/m2, and Io is at 2500 mW/m2. Earth’s Moon, meanwhile, is at a paltry 9 to 13 mW/m2 from radiogenic decay.

Hand then explained how plumes can allow for easy confirmation of a subsurface, liquid water ocean. A recent Hubble image revealed a possible liquid water plume around Europa. Enceladus, on the other hand, has an

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

ocean that has been confirmed by the Cassini spacecraft’s discovery and fly-throughs of its plume. Enceladus’s tidal energy is likely due to the 2:1 tidal resonance it has with the more distant Saturnian moon Dione, although as mentioned before, the long-term nature of this tidal heating is debated.

The Availability of Elements

Another keystone for life, Hand suggested, is the availability of elements necessary for building life (CHNOPS [carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur] and some metals). These ocean worlds formed in the outer solar systems where the condensing volatiles contained a large quantity of the CHNOPS elements. The interiors and surfaces of the icy worlds therefore retained a lot of these key molecules and elements. Hand noted, ironically, that it is difficult to explain exactly how Earth got so much water and carbon.

In terms of heavier elements, a simple check of bulk density can be informative (see Figure 2.3). Io and Europa (and the Moon) are predominantly rocky and likely silica-rich, with densities ≳3,000 kg/m3, while all other icy ocean worlds have densities in the approximate range of 1,000 to 2,000 kg/m3. The larger bodies (Ganymede, Titan, Callisto, and maybe Triton and Pluto) are massive enough that there is a phase transition to a denser ice (ice III, ice IV, and/or ice VI) that sinks and lines the ocean floor, hindering or altogether preventing water-rock interactions on their seafloors.

Hand said that both Europa and Enceladus have been shown to contain heavier elements. He highlighted the detection of silica in the ice grains of Saturn’s E-ring (created by Enceladus) by Cassini’s Cosmic Dust Analyzer (CDA). This implies a low-temperature, alkaline, water-rock interaction that provides ~200 ppm of silica to the ocean. On Europa, the discoloration on its surface is believed by many to be salt from within its ocean. Hand, however, had not been fully convinced by the limited spectra supporting the salt hypothesis, instead having largely preferred the sulfuric acid hypothesis, which said that the discoloration is primarily sulfuric acid on the surface derived from sulfur originating from Io’s volcanism. At JPL, Hand has a laboratory “Europa-in-a-can” to test for the source of the discoloration. They introduced salt to the sample of artificial Europan material as an evaporite and irradiated it with an electron gun at 10 keV. This turned the white salt into a yellowish brown, which is evidence

FIGURE 2.3 Density of solar system bodies compared to radius. SOURCE: Reprinted from H. Hussmann, F. Sohl, and T. Spohn, 2006, Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects, Icarus 185(1):258-273, copyright 2006, with permission from Elsevier; presented in Kevin Hand, Jet Propulsion Laboratory, “On the Habitability of Ocean Worlds,” presentation to the Workshop on Searching for Life across Space and Time, December 5, 2016.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

of the so-called F and M color centers that arise when defects are formed in crystals due to the presence of trapped electrons in negative (i.e., anion) vacancies. Hand now predicts that, in certain regions on Europa, the discoloration is from salts coming up from the ocean and being irradiated and discolored. Keck telescope observations show a spectrally unique region in the Powys Regio. Hubble will soon be looking at this region to check for irradiated salts.

Energetics Needed for Life

Hand then moved on to the energy needed for life to grow, reproduce, and metabolize. An active seafloor, he said, is not sufficient for chemosynthesis if there is not the right combination of electron donors and acceptors. He argued that the radiation environment of Europa, coupled with geologic overturn of the ice shell, could solve that problem. The radiation has made Europa’s surface covered with oxidants. Charged particles split water and create OH radicals, which then combine to make hydrogen peroxide (H2O2). Some peroxide stays around, some decays (with H2 escape), and some combines with sulfur. This leads to a surface rich in peroxide, oxygen, sulfate, sulfur dioxide, carbon dioxide, and more. Geological activity could then introduce these molecules into Europa’s oceans at a rate that could sustain an active subsurface biosphere within Europa’s ocean.

Potential Biosignatures on Europa

Hand then used a thick ice shell model (~15 km) for Europa, on top of a thick ocean of water (~100 km), to examine exchange processes and how potential biosignatures might be preserved on the surface of Europa (see Figure 2.4). The top layer is composed of brittle ice. Underneath this surface is a layer of ductile, convective ice.

FIGURE 2.4 Possible habitable regions, sites of biosignature preservation, and sites of potential biosignature emplacement on Europa. SOURCE: Kevin Hand, Jet Propulsion Laboratory, “On the Habitability of Ocean Worlds,” presentation to the Workshop on Searching for Life across Space and Time, December 5, 2016.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

He said that the seafloor region could potentially be habitable, but that the ice-water interface could also be a chemically rich and potentially habitable interface. Oxidants from the surface could mix with reductants delivered from ocean currents. Fractures and diapirs could provide pathways to deliver material up or down. This could lead to several regions within or at the boundaries of the icy crust that could be habitable.

Hand finished by talking about the possible conditions that lead to the origin of life and how biology might someday be found to have an organizing principle similar to the periodic table of the elements or the Gibbs phase rule. Hand said that revealing these organizing principles, and revolutionizing our understanding of biology, would require finding a second, independent origin of life. The ocean worlds of the outer solar system provide prime locales for testing the biology hypothesis and whether or not biology works beyond Earth.

Audience Participation

A member of the audience commented that Hand provided an explanation of Europa’s surface discoloration that does not require minerals, only salt. Therefore, it does not necessarily have to be from the solid core. He then asked what the latest thinking was on where the energy from tidal forcing is being deposited: Europa’s core or its water-ice interface? Hand said that there is still debate about where the energy is deposited but that the key is the partitioning in the mantle or the ice shell. The Europa Clipper (previously known as the Europa Multiple-Flyby Mission) could help reveal this partitioning.

After getting confirmation that the salt irradiation experiment at JPL used only electron radiation, an audience member then asked if the energy flux of the electron radiation was similar to the modern day energy flux on Europa or if it was adjusted to account for historically variable flux levels. Hand said that the total fluence (flux integrated over time) into the salts maps quite well with reality. Furthermore, the reason they focused only on electron radiation is that 80 percent of Europa’s radiation comes from energetic electrons in Jupiter’s magnetosphere.

The same audience member then asked a different question related to possible hydrothermal activity on Europa. She asked how the chemistry and energy from the hydrothermal vents could be carried through the entire thick ocean to the water-ice interface at the crust. For example, she said, you can’t detect the hydrothermal vent systems on Earth at the ocean surface. Hand answered that modeling has looked at what happens to buoyant plumes under different conditions. Results show that plumes could stay contained by traveling vertically through tens of kilometers of ocean—and perhaps all the way to the water-ice interface. He then said that work on Earth is attempting to measure how high Earth’s hydrothermal plumes rise in a coherent fashion. The canonical answer, he said, is about 400 to 600 m. They looked for, but did not find, any evidence in the Arctic ice. Another audience member then said that, even if a plume cannot stay self-contained up to the ice-water interface, the by-products could still be delivered through ocean mixing. She then noted that plumes would be the most stable at the poles. Then another audience member went further and showed how hydrothermal systems can influence the whole ocean. He said that a 2015 paper indicated that the overwhelming amount of iron in Earth’s ocean comes from hydrothermal systems. Models of trace elements, such as molybdenum, are also starting to show the same thing. He then pointed out that, while there are only about 40 to 50 hydrothermal vent systems known, global budget models show that there could be as many as 900. Extrapolating the ocean abundance models to include unknown hydrothermal systems suggests that 60 to 70 percent of inorganic nutrients could originate from hydrothermal vents. He then said that, even on Earth, there is not a deep understanding of how circulation happens in the deep oceans. How tidal fluxing could affect these circulation patterns on Europa is also unknown. Hand followed this saying that the SiO2 data from Enceladus, if it is indeed hydrothermal, must have traveled up through at least 50 km of ocean water and 30 km of ice before it could get into space. Even if these bodies do not have life, there is a tremendous amount that could be learned in terms of comparative oceanography.

A workshop participant then pointed out that contamination from Earth life on probes could be problematic. Hand completely agreed and said that planetary protection needs to be a primary concern.

Another participant at the workshop then asked whether a fly-by mission of Enceladus might actually be able to capture cells. She then asked if spores in the plume could travel to other nearby bodies. Hand said that any cells ejected from a plume would likely die after exposure to the space environment. Any cell that gets on the surface would first freeze, which would actually be good in terms of organics and biosignature preservation. However, it

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

would then be irradiated, which could destroy life and alter biosignatures. On the other hand, in the Alum Shale Formation in southern Scandinavia, where there are a lot of uranium-rich materials, strong organic biosignatures are present, despite all of the radiation processing. Changing topics to Europa, Hand finished by saying that, even in its harsh radiation environment, he still thinks that biosignatures in the oceans and the ice could be preserved and detectable.

Ellen Stofan, the former chief scientist at NASA, began her talk by describing the need for NASA to incorporate research from all of its main scientific fields (astrophysics, heliophysics, planetary science, Earth science, and microgravity) and also from those fields outside of NASA’s purview in order to clearly understand what is needed in the search for extraterrestrial life. A recent breakthrough, she said, is the exoplanet revolution due to the Kepler space telescope’s discoveries. One of its major discoveries was the huge number of exoplanet candidates in the super Earth to sub-Neptune range. The Transiting Exoplanet Survey Satellite (TESS; expected launch 2018) will build on these discoveries.

The exoplanet revolution has led to a re-examination of what the habitable zone is, both within our solar system and beyond. Stofan emphasized the fact that the habitable zone is not just dependent on spatial location, but also dependent on time. Venus in its early history may have been habitable. Earth’s twin almost certainly lost an ocean’s worth of water in the past, which may have been stable on its surface before being lost to a runaway greenhouse effect. Two Phase A concepts for going back to Venus were in the works to help answer important questions about Venus, such as the compositions of the atmosphere and the rocky surface, the isotopic composition of the atmosphere, and the minerology of its surface rocks, but neither was ultimately selected. New Frontiers will be the next chance for Venus missions.

For Mars, the history of water on the planet is key, Stofan said. Water was stable on Mars’ surface for a long period of time, raising the prospects for past habitability on Mars. She thinks that humans are going to need to land on Mars to fully explore the planet and its potential for past life. This includes drilling deep (below 2 m) and getting samples from multiple locations. Stofan is optimistic about sending humans to Mars. Research on the International Space Station (ISS) has been done to figure out how to keep astronauts healthy for long periods of time spent in space. A plan has been made for using the ISS to investigate how to mitigate bone density loss, muscle wasting, and decreased immune system functioning. Life support systems are also critical. Recycling water, keeping CO2 levels down, and just keeping the toilets working are all necessary for a successful Mars mission. By the mid-2020s, they plan to put a prototype of the Mars transfer vehicle into orbit around the Moon to test the environmental control and life support systems as a concept demonstration for a crewed Mars mission.

NASA plans to continue sending robotic rover missions to Mars, such as the Mars 2020 rover. In preparation for human missions to Mars, NASA also plans to do robotic landing missions in the late 2020s to test equipment and procedures for future crewed attempts, Stofan said. A crewed martian orbiter mission is planned for 2033. On this mission, they would like to tele-operate a rover on Mars, perhaps going to a region that NASA does not want to send humans. The mission to land the first humans on Mars is slated for the late 2030s. NASA is also partnered with SpaceX for Red Dragon, a planned low-cost, robotic martian lander planning to launch in 2018 or 2020. Planetary protection, however, is a major concern. A decade-old study said it was necessary to do a sample return mission before sending humans, and Stofan thinks that this topic needs to be revisited. Stofan said that a National Academies of Sciences, Engineering, and Medicine study might be useful to determine the best approach.

Stofan then pivoted to the solar system’s present (and potential, not yet fully described) Ocean Worlds. The possible plumes on Europa are of particular interest and have sparked greater interest in a Europa lander to be added onto a future orbital mission. A current Jovian mission, the Europa Clipper, is already planned for a launch in the early to mid-2020s. Stofan is excited about the possibility of using the Space Launch System, which cuts travel times to the outer solar system approximately in half, returning data in a much more timely manner (i.e., before the science teams retire).

Stofan mentioned the only currently approved missions related to the exploration of extraterrestrial life in the solar system are the Mars 2020 rover and the Europa Clipper. A Europa lander mission is being extensively

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

studied at the moment too. On December 9, 2016, NASA announced a call for New Frontiers missions, which now includes a category for oceans worlds (although only for Titan and/or Enceladus at the moment). Stofan finished by saying that she hopes that a robotic demonstration mission that includes sample return in the late 2020s will pave the way for a crewed mission in the 2030s to land scientists on the martian surface to explore the possibility of alien life on Mars.

Audience Participation

A member of the audience commented that he didn’t hear anything about searching for extant life on Mars in Stofan’s talk. He said that this might be the first thing we want to do, especially before sending humans, which could possibly contaminate the martian surface. Stofan agreed that it was a good point and again said that a study by the National Academies is needed to determine the necessity of doing a sample return mission before sending humans.

Another member of the audience asked whether the evolution of our understanding of rocky exoplanets changes what we are planning to look for in our own solar system’s rocky planets. Stofan said that it changes the questions that are being asked. Looking at other planetary systems, she thinks that we need to better understand our own solar system’s habitable (or once habitable) rocky planets: Venus, Earth, and Mars. She said that there are fairly straightforward missions and measurements that can be done, especially for Venus, that have just been ignored.

Several other entities, both state and private, are planning to go to Mars, one audience member said, specifically mentioning India, Europe, Russia, and China. He then asked whether NASA feels that there is a soft space race for going to Mars and whether the United States and its political leaders are aware of that. Stofan mentioned a recent article asking whether NASA was really going to Mars or if it would pivot away. The article also quoted someone saying that there isn’t a reason to send people to Mars. Stofan strongly disagreed with that. She also doesn’t see the push for Mars as a new, soft space race, but rather, a confluence of interests, including major public interest. She said that sending humans to Mars garners the most public reaction and interest out of all of NASA’s projects. She also said that a crewed mission to Mars can be done in collaboration with other countries and with private firms. Mars, she said, will be done and made affordable by cooperation and collaboration rather than competition.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

Suggested Citation:"2 Habitable Environments in the Solar System." National Academies of Sciences, Engineering, and Medicine. 2017. Searching for Life Across Space and Time: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/24860.

The search for life is one of the most active fields in space science and involves a wide variety of scientific disciplines, including planetary science, astronomy and astrophysics, chemistry, biology, chemistry, and geoscience. In December 2016, the Space Studies Board hosted a workshop to explore the possibility of habitable environments in the solar system and in exoplanets, techniques for detecting life, and the instrumentation used. This publication summarizes the presentations and discussions from the workshop.

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